1. Field of the Invention
This invention relates to a process for producing .gamma.-cyclodextrin without using any organic solvents i.e. by non-solvent process and more particularly, to processes for producing .gamma.-cyclodextrin, which comprise: (1) passing a sugar solution containing cyclodextrins and reducing sugars as primary ingredients through a column packed with alkali or alkali earth metal salts of strongly acidic cation exchange resin to separate cyclodextrins fraction from reducing sugars fraction and then, passing the cyclodextrins fraction through a column packed with gel resin particles to separate and collect .gamma.-cyclodextrin and, (2) bringing the sugar solution into contact with a hydrophobic, synthetic adsorption resin comprising a porous polymer to adsorb only .gamma.-cyclodextrin or .gamma.- and .beta.-cyclodextrins and then, eluting the adsorbed cyclodextrin(s) with water to separate and collect .gamma.-cyclodextrin.
2. Description of the Prior Art
Cyclodextrins are non-reducing dextrins which have a ring structure wherein glucose units are joined to one another by .alpha.-1,4-glucosidic linkages. Generally, .alpha.-cyclodextrin, .beta.-cyclodextrin and .gamma.-cyclodextrin, which consist of 6, 7 and 8 glucose units, respectively, are well known. The cyclodextrin molecule includes various kinds of organic compounds into the cavity of the ring structure which is in a hydrophobic atmosphere to form an inclusion compound. Alpha-, beta- and gamma-cyclodextrins have many different physical properties from one another: the molecular weights (.alpha.=973, .beta.=1135 and .gamma.=1297), the cavity diameters of ring structure (.alpha.=5-6 .ANG., .beta.=7-8 .ANG. and .gamma.=9-10 .ANG.) and water solubility (at 25.degree. C., g/100 ml.H.sub.2 O: .alpha.=14.5, .beta.=1.85 and .gamma.=23.2)
Accordingly, cyclodextrins find wide use in medicines, agricultural chemicals, cosmetics and foods and new applications thereof are expected to be found. Only a mixture of .alpha.-, .beta.- and .gamma.-cyclodextrins and starch hydrolyzate containing acyclic dextrins and .beta.-cyclodextrin precipitated and separated from such mixture, are products which can be produced, at present, at relatively low cost. Alpha-cyclodextrin and .gamma.-cyclodextrin can not be produced at low cost. The industrial production of .alpha.- and .gamma.-cyclodextrins is deemed difficult, although they have special properties and their specific applications are expected as described above. This is due to the difference of water solubility of .alpha.-, .beta.- and .gamma.-cyclodextrins. Namely, the water-solubility of .beta.-cyclodextrin is very low compared with .alpha.- and .gamma.-cyclodextrins and thereofore, it is easily precipitated and separated from the starch hydrolyzate solution containing cyclodextrins. On the contrary, .alpha.- and .gamma.-cyclodextrins have relatively high water solubilities and so, it is difficult without using any organic solvents to separate these cyclodextrins as crystal from the starch hydrolyzate solution containing cyclodextrins.
There are many proposed processes for producing cyclodextrins hereinbefore. One typical process is such that liquefied starch products are reacted with cyclodextrin glycosyltransferase, hereinafter refer to "CGTase," to obtain starch hydrolyzate solution containing cyclodextrins followed by adding an organic solvent such as trichloroethylene, tetrachloroethane or bromobenzene to precipitate cyclodextrins as a complex and to separate cyclodextrins from the starch hydrolyzate.
An improved process of the typical one mentioned above has been proposed, which comprises reacting the starch hydrolyzate containing cyclodextrins with glucoamylase in order to hydrolyse acyclic dextrins to convert them into glucose and then adding an organic solvent to the resulting sugar solution containing cyclodextrins and glucose to precipitate only cyclodextrins (see DENPUN-KAGAKU (Starch Science) 22, pp. 6-10 (1975): Japanese Pat. No. 1,034,493).
However, organic solvents are used to precipitate cyclodextrins in all the aforementioned processes and contaminate the cyclodextrins thus obtained so that they cannot be utilized in the production of medicines or foods due to the toxicity of the solvents and the production cost should have been high.
Some processes for producing cyclodextrins without using any organic solvents, that is, non-solvent processes have recently been discussed and proposed. There are proposed two non-solvent processes, that is, the one comprising reacting liquefied starch with an enzyme having an optimum pH on the alkaline side instead of CGTase, concentrating the reaction mixture and adding a small amount of cyclodextrin as a seed crystal to precipitate cyclodextrins (see Japanese Pat. No. 914,137) and the other comprising contacting a solution containing cyclodextrins and reducing sugars with OH type of anion exchange resin to adsorb only reducing sugars on the resin to thereby separate cyclodextrins from the reducing sugars (see Japanese Patent Disclosure No. 136,889/1976).
These non-solvent processes are advantageous since cyclodextrins obtained by the processes have no toxicity of the solvents so that they can widely be used in the field of medicines or foods. However, these processes have some disadvantages: the former process requires complicated operations and the latter process wherein OH type of anion exchange resin is used is not sufficient to effectively and continuously produce cyclodextrins because elution rate of reducing sugars adsorbed on the resin is too low, and if HCl is used to increase the elution rate, a regeneration of the resin after HCl elution is required and further, a temperature of liquid passing through the column packed with the resin should be controlled below 20.degree. C. in order to prevent reducing sugars adsorbed from decomposing. In addition, the aforementioned non-solvent processes are those for producing .beta.-cyclodextrin or a mixture of .alpha.-, .beta.- and .gamma.-cyclodextrins but are not those for selectively separating .gamma.-cyclodextrin or .gamma.-cyclodextrin.
The inventors have proposed a process for producing cyclodextrins, which comprise passing a sugar solution containing cyclodextrins and reducing sugars as primary ingredients through a column packed with alkali or alkali earth metal salts of strongly acidic cation exchanged resin to separate and collect cyclodextrins with industrial and economic advantages (see the specification of Japanese Patent Application No. 104,383/1980). However, the product obtained by this process is a mixture of .alpha.-, .beta.- and .gamma.-cyclodextrins or .alpha.-cyclodextrin. Gamma-cyclodextrin could not selectively produced by the above mentioned process. It is known that a solution obtained by dissolving crystalline .alpha.-, .beta.- and .gamma.-cyclodextrins is passed through a column packed with gel resin particles, each fraction of .alpha.-, .beta.- and .gamma.-cyclodextrins can be separated on a laboratory scale. It is, however, not known at all that starch hydrolyzate containing cyclodextrins obtained by reacting liquefied or gelatinized starch with an enzyme is subjected to a gel filtration to separate and collect .gamma.-cyclodextrin and such a process has been deemed impossible on an industrial scale.
As stated above, cyclodextrins which have special molecular structure and properties and from which possible utilization, in particular, .gamma.-cyclodextrin which has a high water solubility and a large cavity diameter of the molecule, may be extremely be useful, if cyclodextrins or .gamma.-cyclodextrin could be produced with industrial and economic advantages.